The present invention relates to a method and apparatus for cleaning surfaces, including metals and plastics, to a higher level of cleanliness than is possible with conventional solvents, acid baths, or detergents. More specifically, the present invention is directed to the cleaning of surfaces using the active species from a glow discharge plasma which is capable of operating at one atmosphere of pressure.
It has been found that plasma-cleaned surfaces can provide a substrate for more corrosion-resistant electroplated surfaces, greater adhesion of electroplated layers, greater adhesion of paint to surfaces, stronger adhesive bonding of surfaces, and many other industrial applications in which surface cleanliness is a factor, including the sterilization of surfaces. Operation of the plasma-cleaning process at one atmosphere would eliminate the need for costly vacuum systems and batch processing associated with presently existing low pressure glow discharge and plasma processing used industrially.
In the physical sciences, the term "plasma" describes a partially or fully ionized gas, which is sometimes referred to as a "fourth state of matter". Industrial plasmas, such as are described herein, are partially ionized and consist of ions, electrons and neutral species. This state of matter can be produced by the action of very high temperatures, or strong direct current (DC) or radio frequency (RF) electric fields.
High energy density, high intensity plasmas are represented by stars, nuclear explosions, plasma torches and electrical arcs. Medium energy density, glow discharge plasmas are produced by free electrons which are energized by an imposed DC or RF electric field, causing collisions with neutral molecules. These collisions with neutral molecules transfer energy to the molecules and to the ions, forming a variety of active species which may include photons (both ultraviolet and visible), excited atoms and molecules, metastables, individual atoms, free radicals, molecular fragments, monomers, electrons and ions. These active species can have energies, or be in excited energy states, of several or even several tens of electron volts (one electron volt approximates 11,600.degree. K). This is far higher than the chemical binding energy associated, for example, with chemical cleaning processes. These active species can then form a more energetic medium for plasma cleaning (to effectively remove more tightly bound monolayers of dirt and contaminants on surfaces) than can be reached with less energetic chemical cleaning processes. Low power density, low intensity plasmas, such as dark discharges and coronas, have been used at low pressure and at one atmosphere for the surface treatment of various materials. However, because of their relatively low energy density, corona discharges can alter many surface properties of materials only relatively slowly, if at all.
The use of corona discharges for surface cleaning is, in general, unsatisfactory because of their low power density; and filamentary discharges of moderate average power density, because of their non-uniformity of effect. The use of arcs or plasma torches is also unsatisfactory because their energy densities are sufficiently high to damage many treated materials.
In many applications, glow discharge plasmas have the advantage of providing enough active species to produce important effects, but are not of sufficiently high intensity to damage the surface being treated. However, it is important to note that glow discharge plasmas have heretofore typically been successfully generated in low pressure or partial vacuum environments (e.g., below 10 torr). This usually necessitates a batch processing of treated materials, and the use of vacuum systems, which are expensive to buy and maintain.
An essential feature of the present invention is to be able to plasma-clean surfaces at one atmosphere. Preferably, this is accomplished using a uniform glow discharge plasma capable of operating at one atmosphere, and at moderate plasma power densities which will provide an adequate intensity of active species without damaging the surface to be treated. The generation of low, moderate and high power density plasmas at one atmosphere is generally known. Such plasmas may be generated, for example, using the one atmosphere uniform glow discharge plasma reactors described in U.S. Pat. Nos. 5,387,842, 5,403,453 and 5,414,324, as well as U.S. patent applications Ser. No. 08/068,739 (filed May 28, 1993) and 08/254,264 (filed Jun. 6, 1994), the subject matter of each of which is fully incorporated herein by reference.